A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus using very short wavelength exposure radiation, e.g. EUV in the range of 5 to 20 nm, the beam path should be kept at a high vacuum or a low pressure, e.g. 10−3 mbar, of Argon, as the transmission of air at standard atmospheric pressure at such wavelengths is very low. To prevent particles from settling onto the mask, a so-called electrostatic pellicle that includes a grid of wires at a high voltage, e.g. 1000V, relative to the mask has been proposed. The electrostatic pellicle attracts particles, especially ions, approaching the mask, and prevents them settling thereon. A similar arrangement has been proposed to protect the mirrors in the illumination and projection system from damage and is disclosed in EP-1 182 510-A, as well as unpublished European Patent Applications Nos. 02079329.5 and 02079864.1, the contents of which are all incorporated herein by reference.
With any high voltage system, there is a risk of breakdown, especially during periods when the pressure in the system is rising or falling. As shown in FIG. 2 of the accompanying drawings, which shows breakdown potential difference vs. the product of pressure and distance (P.d), the breakdown voltage is high at low and high values of P.d, but has a minimum at about 1 mb.cm. The value of the minimum depends on what current flow is considered to be a breakdown, but for present purposes may be considered to be about 300V. Thus there is a risk of breakdown during pump down or when the system is returning to atmospheric pressure if high voltage systems, such as the electrostatic pellicle, are still operating. While these systems can normally be switched off during planned pressure changes, given the expense of masks and mirrors in a lithographic apparatus, it is desirable to provide protection to prevent damage that might be caused by a breakdown during an unplanned pressure change, such as one caused by a valve or software failure.